1. Technical Field of the Invention
The present invention relates generally to the field of optical inspection systems. More particularly, the present invention relates to systems and methods for reconstructing three-dimensional images of specular objects for printed circuit board and electronics manufacturing inspection.
2. Description of Related Art
In conventional optical inspection systems, the data collected produce a two-dimensional image of a three-dimensional object. For example, in optical inspection of printed circuit boards (PCBs), the data collected produce a two-dimensional image of the object surface. Although a number of techniques for reconstructing a three-dimensional image from multiple two-dimensional images are known in the computer vision industry, these techniques have not been applied to the printed circuit board optical inspection industry due to the difficulty in processing the specular reflections off shiny specular objects, such as solder joints and other components present on PCBs.
For example, one method of reconstructing a three-dimensional image is referred to as a “photometric” stereo method, which separates the specular and diffuse reflections and derives surface gradients for surfaces that have both specular and diffuse elements by combining information from several images of the same target taken with different illumination angles. However, photometric stereo methods have not been applied to optical inspection of mostly specular objects (e.g., solder joints on printed circuit boards) due to the additional processing required to analyze both the specular and diffuse reflections, and therefore the additional time required for each inspection.
Thus, in conventional optical inspection systems, a determination of whether an object is defective is made by manually analyzing the two-dimensional image of the object surface. The manual inspection process is labor-intensive and prone to errors. In many cases, two-dimensional images do not provide enough information to accurately assess whether or not an object is defective.
Therefore, what is needed is an optical inspection system capable of reconstructing a three-dimensional image of the surface of a specular object. In addition, what is needed is an optical inspection system capable of processing both the specular and diffuse reflections to reconstruct the three-dimensional image.